Spinning polyacrylonitrile filaments and washing the filaments in contact with an absrbent surface



y 1962 P. HALBlG ETAL 3,035,884

- SPINNING POLYACRYLONITRILE FILAMENTS AND WASHING THE FILAMENTS IN CONTACT WITH AN ABSORBENT SURFACE Filed June a, 1959 United States Patent 3,035,884 SPINNING POLYACRYLONITRILE FILAMENTS AND WASHING THE FILAMENTS IN CON- TACT WITH AN ABSORBENT SURFACE Paul Halbig and Camille Nordmann, Fribonrg, Switzerland, assignors to Lonza Electric and Chemical Works Ltd., Basel, Switzerland Filed June 3, 1959, Ser. No. 817,876 Claims priority, application Switzerland June 6, 1958 2 Claims. (Cl. 18-54) The invention relates to the production of filaments by the wet spinning process from polymers which consist entirely of polyacrylonitrile, or of copolymers which contain at least 85 percent by weight of acrylonitrile monomer units, the balance being polymers of compounds polymerizable with acrylonitrile. In the following description, the term polyacrylonitrile is used to include such polymers and copolymers.

More particularly, the invention relates to improvements in the spinning of nitric acid solutions of polyacrylonitrile. Such solutions contain the polyacrylonitrile dissolved in nitric acid of more than 52% and are extruded into a spinning bath which consists of aqueous nitric acid containing less than 50% HNO It is a principal object of the invention to provide a method of recovering the nitric acid adhered to the filaments in a concentration which can directly be re-used as spinning bath.

It is another object of the invention to remove the nitric acid without affecting the properties of the spun filaments.

Other objects and advantages will be apparent from a consideration of the specification and claims.

While the spinning 'of polyacrylonitrile filaments from concentrated sulfuric acid solutions had to be abandoned because of the difliculties of removing the acid from the filaments by washing, we have found that nitric acid can be completely separated form the coagulated polyacrylonitrile already by surprisingly small amounts of water.

Particularly, we have discovered that wash water can be applied in only such small amounts as required to maintain a uniform concentration of the spinning bath, whereby almost the entire nitric acid entrained by the filaments can be returned to the spinning bath in a continuous and fast operation readily incorporated in the overall spinning process. In this way, substantially the entire amount of solvent is recovered in the highest possible concentration for its repeated use, and the filaments are quickly, completely and very gently deacidified.

The required amount of wash water is readily calculated from the concentrations of the spinning solution and co agulating bath, taking into account the amount of water entrained by the acid-free washed filaments. The concentration of the polymer in the spinning solution is practically between 8 and 14 percent, the concentration of HNO in the solvent, which consists of H and HNO is between 52% and 65%, preferably 58 to 62%, and the concentration of HNO in the coagulating bath is between 50% and 40%, preferably 43 to 46%. The Washed filaments entrain an amount of water corresponding to about 500 to 700 percent of the weight of the filament.

In the following table, we set out the required amount of wash water in percent by weight of the dry filaments for spinning solutions containing 8%, 12% and 14% of polymer, respectively. The HNO concentration of the nitric acid solvent is assumed to be 59%, that of the coagulating bath 44%, and the amount of water entrained by the filaments is 500 to 700%.

0 ice Amount of solvent for Required amount Concentration 01 polymer of wash water in The table shows that the required amount of wash water must be of the same order of magnitude as the weight of the spinning solution. Therefore, this amount is relatively small, and it is quite unexpected, particularly in view of the experiences made with sulfuric acid, that such a small water amount is suflicient to recover substantially the entire solvent in the same concentration as the coagulating bath.

In carrying out the invention, a continuous uniform and eflicient distribution of the wash water on the filaments countercurrentwise is necessary. This distribution is accomplished by passing the filaments in helical turns over a vertical pair of rollers, which consists of the wellknown combination of a wash roll proper with a threadadvancing auxiliary roll. The rolls are equipped with an absorbent and readily wetted surface. Such surface may be readily provided by means of a suitable fabric, felt or fibrous fleece. Polyvinyl chloride is a suitable material for such surface layers. The coagulated filaments pass upwardly around said roller pair in countercurrent to the water supplied to the top of the rollers. Such a device is simple and does not afiect the filaments because it eliminates the otherwise used doctor blades and squeezing means for squeezing out the liquid.

A suitable apparatus for carrying out the invention is shown, by way of example, in the accompanying diagrammatic drawing.

In the drawing, the reference numeral 1 designates a vessel containing the acid which is pumped by pump 2 through line 10 into the vessel 3 containing the spinning bath, from which it returns through the overflow 11 into reservoir 1. Excess coagulating acid drains off through overflow 12. Said acid may be upgraded to the concentration required for the solvent nitric acid, for instance by fractional distillation.

The required amount of water is added through the liter counter '13 on the washing device 7, which consists of two polyvinyl chloride rolls and a collecting dish 14 for the drained wash liquid. Both rolls are coated with a polyvinyl chloride fabric 15 of about 0.4 mm. thickness. The washing liquid flows from the dish 14 through conduit 11, where it is mixed with the acid from the coagulating bath, into the reservoir 1. The washing device 8 is similar to device 7 and serves to rinse out from the filaments the last traces of nitric acid with suflicient water metered by the counter 17. The liquid flowing from the dish 16 is collected in the vessel 9. It contains only traces of HNO' and can, therefore, be discarded. However, it is important to test the nitric acid content of said washing liquid as a control of the satisfactory operation of the washing device 7.

The operation of the apparatus may be as follows: A spinning solution is prepared, which consists of 120 parts by weight of polyacrylonitnile and 880 parts by Weight of 59% nitric acid. 600 g./ hour of said solution, which contain 72 g. of polymer, 312 g. of HNO and 2116 g. of water, are extruded through the spinneret 4, which has 100 holes of 0.4 mm. diameter, into the coagulating bath 3, in which aqueous nitric acid containing 44i0.5% HNO circulates at a temperature of -2 to +6 C. About 10 cm. behind the spinneret, the coagulated filaments are received by the drawing rolls 5 and subsequently by the second drawing roller pair 6 and the Washing devices 7 and 8. The rate of travel of the filaments in the devices is adjusted to 3.5 m./min. on 5, 13.0 m./min. on 6, and 13.5 m./min. on 7 and 8. The filaments leaving 8 are then processed in the conventional stretching, cutting, shrinking, and other devices.

By means of the metering device 13, about 600 g./hour are uniformly fed to the top of the wash roll. The acid concentration of the coagulating bath is continuously checked'by determination of the specific gravity. If the concentration rises above 44.2% HNO the water amount is slightly increased, for instance to about 620 g.; if the concentration falls below 43.8%, the water amount is slightly reduced, for instance to 580 g. In this way the concentration of the coagulating bath is readily kept within 44:0.5. About 700 g./hour of 44% nitric acid lflow off through the overflow 12 and are passed to distillation. 1

On the washing device 8, the filament is washed in the same way as on 7 but with 1200 g. of water per hour. The nitric acid content of the wash liquid leaving the device 8 is only 0.9 to 2.0 g. per hour, corresponding to 0.29 to 0.64 percent of the nitric acid amount contained in the solution of the polymer. When leaving the device 8, the filament is entirely free of nitric acid. The solvent is,

therefore, substantially completely recovered in the concentration of the spinning bath.

The etfect of the absorbent surface of the wash 'rolls is illustrated by the following comparative tests, carried out as set forth hereinabove. In test A, the pair of rolls consisted of two polyvinyl chloride rolls, the surface of which had been made water repellant by a single rubbing with silicone oil. For test B, the same rolls had been made water wcttable by a single treatment with sodium dodecylsulfonate. In test C, the rolls had been covered with a polyvinyl chloride fabric of about 0.4 mm. thickness. The following table gives the number of grams of 100% nitric acid found after conversion of 600 g. of spinning solution each to 72 g. of filament within 1 hour in the wash liquid of the Wash device 9.

4 Test A--86.00 g. HNo Test B6.20 g. HNOS Test c 1.15 g. HNOS The outstanding effect of Test C is due to the absorbent cover, which apparently causes a more uniform distribution of the wash water on the filament.

We claim:

1. A method of producing polyacrylonitrile filaments comprising spinning a solution of a polymer selected from the group consisting of polyacrylonitrile and copolymers containing at least 85 percent by weight of acrylonitrile monomer units, the balance being polymers of compounds copolymerizable with acrylonitrile, in aqueous nitric acid containing 52 to 65 percent by weight of HNO into a coagulating bath formed by an aqueous nitric acid solution containing about to percent by weight of HNO washing the newly formed filaments in contact with an absorbent surface continuously and countercurrently with an amount of water just sufiicient to dilute the nitric acid washed out of said filaments to substantially .the concentration of said coagulating bath, and recycling said washed out acid into said coagulating bath, thereby recovering substantially the entire amount of the nitric acid introduced as solvent of said polymer in the form of about 40 to 50% nitric acid directly available for said coagulating bath.

2. The method as defined in claim 1 wherein the newly formed filaments are washed on rollshaving a water absorbent surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,436,028 Wiegerink' Feb. 17, 1948 2,562,199 McLellan et al. July 31, 1951 2,593,555 Guy Apr. 22, 1952 2,723,900 Hooper Nov. 15, 1955 2,730,889 Mayner Jan. 17, 1956 2,871,090 Mayner Jan. 27, 1959 2,878,097 Halbig Mar. 17, 1959 

1. A METHOD OF PRODUCING POLYACRYLONITRILE FILAMENTS COMPRISING SPINNING A SOLUTION OF A POLYMER SELECTED FROM THE GROUP CONSISTING OF POLYACRYLONTRILE AND COPOLYMERS CONTAINING AT LEAST 85 PERCENT BY WEIGHT OF ACRYLONITRILE MONOMER UNITS, THE BALANCE BEING POLYMERS OF COMPOUNDS COPOLYMERIZABLE WITH ACRYLONITRILE, IN AQUEOUS NITRIC ACID CONTAINING 50 TO 65 PERCENT BY WEIGHT OF HNO3 INTO A COAGULATING BATH FORMED BY AN AQUEOUS NITRIC ACD SOLUTION CONTAINING ABOUT 40 TO 50 PERCENT BY WEIGHT OF HNO3, WASHING THE NEWLY FORMED FILAMENTS IN CONTACT WITH AN ABSORBENT SURFACE CONTINUOUSLY AND COUNTERCURRENTLY WITH AN AMOUNT OF WATER JUST SUFFICIENT TO DILUTE THE NITRIC ACID WASHED OUT OF SAID FILAMENTS TO SUBSTANTIALLY THE CONCENTRATION OF SAID COAGULATING BATH, AND RECYCLING SAID WASHED OUT ACID INTO SAID COAGULATING BATH, THEREBY RECOVERING SUBSTANTIALLY THE ENTIRE AMOUNT OF THE NITRIC ACID INTRODUCED AS SOLVENT OF SAID POLYMER IN THE FORM OF ABOUT 40 TO 50% NITRIC ACID DIRECTLY AVALIABLE FOR SAID COAGULATING BATH. 